This invention relates to devices for combining a plurality of digital communication channels.
In telecommunications systems, a large number of communication channels containing voice or data signals may be transmitted together via the same transmission medium, for example, a radio frequency band. A multitude of access schemes for placing communication channels on the transmission medium is known. One class of transmission schemes simultaneously transmits a plurality of different communication channels, e.g., in a radio frequency band, in such a way that they overlap in the time domain as well as in the frequency domain. A well-known access scheme of this class is CDMA (Code Division Multiple Access).
In order to distinguish each communication channel signal from other communication channel signals, each communication channel signal is encoded with one or more unique spreading codes, as it is well-known in the art. For spreading a particular communication channel for transmission, e.g., using CDMA, each symbol of the incoming data stream of the channel, e.g., having a logical value 1 or 0, is represented using the code sequence. For example, if the symbol has the logical value 1, the code sequence itself is transmitted, if the data bit has the logical value 0, then the inverted code sequence is transmitted, or vice versa.
After spreading, the spread communication channels are individually weighted, e.g., according to propagation characteristics and/or the location of the receiving station. Thereafter, the spread and weighted communication channels are combined, i.e., superimposed, to form a single transmission signal, e.g. a CDMA transmission signal, which then may be transmitted via a radio frequency band.
The combining step is usually performed by adding the individual sample values of all data streams of the spread and weighted communication channel signals. Hence, one sample value of the CDMA signal is obtained by summing the respective samples of all individual communication channel signals.
It is conceivable to the person skilled in the art, to add a plurality of individual channels xcfx861-xcfx868 using a cascaded arrangement of adders, as illustrated in FIG. 3. Here, each two input channels are combined by an adder 300 and, in a cascade, the outputs of each pair of adders are combined by a further adder 300. In the shown example, a cascade of three levels of adders generates the transmission signal, e.g., a CDMA signal.
This proceeding may work well for a small number of channels, however, in case of a larger number of channels, a potentially very large number of adders will be needed. In order to reduce hardware costs, alternative solutions for adding up larger numbers of channels are desirable.
It is therefore the object of the invention, to provide a device for combining a plurality of digital communication channels with reduced hardware effort and reduced costs.
This object of the invention is solved by and apparatus for combining, in a plurality of cycles, a plurality of digital communication channels, each being transmitted via communication lines having a first bit width, comprising: a multiplexer for multiplexing the plurality of digital communication channels into a multiplexed digital signal, a register for storing intermediate results, an adder for adding samples of the digital communication channels and intermediate results stored in the register, wherein one of the inputs of the adder is connected to the multiplexer via a connection line having the first bit width, the other input of the adder is connected to the output of the register via a connection line having a second bit width, and the input of the register is connected to the output of the adder via a connection line having the second bit width, and wherein, in a number of cycles corresponding to the number of digital communication channels, one combined output sample is generated by successively adding in each one cycle one sample of one of the communication channels to a previously obtained intermediate result stored in the register.
The object of the invention is further solved by a device for combining, in a plurality of cycles, a plurality of digital communication channels, each being transmitted via communication lines having a first bit width, comprising a cascade arrangement of a plurality of apparatuses, each including: a multiplexer for multiplexing the plurality of digital communication channels into a multiplexed digital signal, a first register for storing intermediate results, an adder for adding samples of the digital communication channels and intermediate results stored in the first register, wherein one of the inputs of the adder is connected to the multiplexer via a first connection line having the first bit width, the output of the adder is connected to the input of the first register via a second connection line having a second bit width larger than the first bit width, and the output of the first register is connected to the other input of the adder via a third connection line having the second bit width, wherein, in a number of cycles corresponding to the number of digital communication channels, one combined output sample is generated by successively adding in each one cycle one sample of one of the communication channels to a previously obtained intermediate result stored in the first register, and wherein the output bit width of an apparatus of a particular stage of the cascade arrangement is determined by ┐lg2(ni(2wixe2x88x921))┌ with ni=number of input signals of the apparatus of a particular stage and wi=bit width of input signals.
According to the invention, the number of adders needed for summing up the plurality of digital communication channels, e.g., to form a digital CDMA transmission signal, may advantageously be reduced.
Advantageously, the second bit width may be the smallest integer equal or larger than a first limit determined by
(lg2xc2x7(number of channelsxc2x7(2first bit widthxe2x88x921))).
Thus, the bit width of the second, third and fourth connection line may be minimized with respect to the bit width of the communication lines of the communication channels.
Further, the second bit width may the smallest integer equal or larger than a second limit determined by
(lg2xc2x7(number of channelsxc2x7(2average signal bit widthxe2x88x921))),
and thus the bit widths of the second, third and fourth connection line may be minimized with respect to the average bit width needed for a representation of the communication signals.
The second bit width may also be an integer equal or smaller than the first limit and equal or larger than the second limit.
Further, according to the invention, a cascade arrangement of combining circuits may be provided. Further advantageous embodiments of the invention are described in further dependent claims.